Systems and methods for improved collimation sensitivity

ABSTRACT

A detector assembly is provided that includes a semiconductor detector, a collimator and a processing unit. The semiconductor detector has a first surface and a second surface opposed to each other. The first surface includes pixels (which in turn comprise corresponding pixelated anodes), and the second surface includes a cathode electrode. The collimator includes openings, with each opening associated with a single corresponding pixel of the semiconductor detector. The processing unit is configured to identify detected events within virtual sub-pixels distributed along a length and width of the semiconductor detector. Each pixel includes (e.g., has associated therewith) a plurality of corresponding virtual sub-pixels, with absorbed photons are counted as events in a corresponding virtual sub-pixel. Absorbed photons are counted as events within a thickness of the semiconductor detector at a distance corresponding to an energy window width used to identify the events as photon impacts.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates generally to apparatus andmethods for diagnostic medical imaging, such as Nuclear Medicine (NM)imaging.

In NM imaging, systems with multiple detectors or detector heads may beused to image a subject, such as to scan a region of interest. Forexample, the detectors may be positioned adjacent the subject to acquireNM data, which is used to generate a three-dimensional (3D) image of thesubject.

Single Photon Emission Computed Tomography (SPECT) systems may havemoving detector heads, such as gamma detectors positioned to focus on aregion of interest. For example, a number of gamma cameras may be moved(e.g., rotated) to different angular positions for acquiring image data.The acquired image data is then used to generate the 3D images.

The size of the detector heads may limit an available usable area forthe placement of detectors, such as Cadmium Zinc Telluride (CZT) wafers.The sensitivity (e.g., the proportion of radiation received relative tothe radiation emitted) may be limited by the size of the detector headsand/or the arrangement of CZT wafers. Conventional approaches toimproving sensitivity may use thicker detectors, or detectors arrangedin generally identical or similar layers stacked directly one on top ofeach other. Such conventional approaches may not provide a desired orrequired sensitivity.

BRIEF DESCRIPTION OF THE INVENTION

In one embodiment, a detector assembly is provided that includes asemiconductor detector, a pinhole collimator, and a processing unit. Thesemiconductor detector has a first surface and a second surface opposedto each other. The first surface includes pixelated anodes, and thesecond surface includes a cathode electrode. The pinhole collimatorincludes an array of pinhole openings corresponding to the pixelatedanodes. Each pinhole opening is associated with a single pixelated anodeof the semiconductor detector, and the area of each pinhole opening issmaller than a corresponding area of the corresponding pixel, which isexposed to radiation (or the pixel area less the radiation blocking areaof the collimator immediately above the pixel). The processing unit isoperably coupled to the semiconductor detector and configured toidentify detected events within virtual sub-pixels distributed along alength and width of the semiconductor detector. Each pixelated anodeincludes (e.g., has associated therewith) a plurality of correspondingvirtual sub-pixels (as interpreted by the processing unit), whereinabsorbed photons are counted as events in a corresponding virtualsub-pixel.

In another embodiment, a detector assembly is provided that includes asemiconductor detector, a collimator, and a processing unit. Thesemiconductor detector has a first surface and a second surface opposedto each other. The first surface includes pixels (which in turn comprisecorresponding pixelated anodes), and the second surface includes acathode electrode. The collimator includes openings. Each opening isassociated with a single corresponding pixelated anode of thesemiconductor detector. The processing unit is configured to identifydetected events within virtual sub-pixels distributed along a length andwidth of the semiconductor detector. Each pixel includes (e.g., hasassociated therewith) a plurality of corresponding virtual sub-pixels.Absorbed photons are counted as events in a corresponding virtualsub-pixel, with absorbed photons counted as events within a thickness ofthe semiconductor detector at a distance corresponding to one over anabsorption coefficient of the detector.

In another embodiment, a detector assembly includes a semiconductordetector, a collimator and a processing unit. The semiconductor detectorhas a first surface and a second surface opposed to each other. Thefirst surface includes pixels (which in turn comprise correspondingpixelated anodes), and the second surface includes a cathode electrode.The collimator includes openings, with each opening associated with asingle corresponding pixel of the semiconductor detector. The processingunit is configured to identify detected events within virtual sub-pixelsdistributed along a length and width of the semiconductor detector. Eachpixel includes (e.g., has associated therewith) a plurality ofcorresponding virtual sub-pixels, with absorbed photons are counted asevents in a corresponding virtual sub-pixel. Absorbed photons arecounted as events within a thickness of the semiconductor detector at adistance corresponding to an energy window width used to identify theevents as photon impacts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a schematic block view of a detector assembly inaccordance with various embodiments.

FIG. 2 provides an exploded view of aspects of the detector assembly ofFIG. 1.

FIG. 3 provides a sectional view taken along line 3-3 of FIG. 1.

FIG. 4 illustrates a cross-section of a top plate formed in accordancewith various embodiments.

FIG. 5 illustrates a cross-section of pinhole collimator septa inaccordance with various embodiments.

FIG. 6 depicts examples of solid angles corresponding to use of aparallel hole collimator.

FIG. 7 is a schematic block diagram of a Nuclear Medicine (NM) imagingsystem in accordance with various embodiments.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of certain embodiments will be betterunderstood when read in conjunction with the appended drawings. To theextent that the figures illustrate diagrams of the functional blocks ofvarious embodiments, the functional blocks are not necessarilyindicative of the division between hardware circuitry. For example, oneor more of the functional blocks (e.g., processors or memories) may beimplemented in a single piece of hardware (e.g., a general purposesignal processor or a block of random access memory, hard disk, or thelike) or multiple pieces of hardware. Similarly, the programs may bestand alone programs, may be incorporated as subroutines in an operatingsystem, may be functions in an installed software package, and the like.It should be understood that the various embodiments are not limited tothe arrangements and instrumentality shown in the drawings.

As used herein, the terms “system,” “unit,” or “module” may include ahardware and/or software system that operates to perform one or morefunctions. For example, a module, unit, or system may include a computerprocessor, controller, or other logic-based device that performsoperations based on instructions stored on a tangible and non-transitorycomputer readable storage medium, such as a computer memory.Alternatively, a module, unit, or system may include a hard-wired devicethat performs operations based on hard-wired logic of the device.Various modules or units shown in the attached figures may represent thehardware that operates based on software or hardwired instructions, thesoftware that directs hardware to perform the operations, or acombination thereof.

“Systems,” “units,” or “modules” may include or represent hardware andassociated instructions (e.g., software stored on a tangible andnon-transitory computer readable storage medium, such as a computer harddrive, ROM, RAM, or the like) that perform one or more operationsdescribed herein. The hardware may include electronic circuits thatinclude and/or are connected to one or more logic-based devices, such asmicroprocessors, processors, controllers, or the like. These devices maybe off-the-shelf devices that are appropriately programmed or instructedto perform operations described herein from the instructions describedabove. Additionally or alternatively, one or more of these devices maybe hard-wired with logic circuits to perform these operations.

As used herein, an element or step recited in the singular and precededwith the word “a” or “an” should be understood as not excluding pluralof said elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” of are not intended to beinterpreted as excluding the existence of additional embodiments thatalso incorporate the recited features. Moreover, unless explicitlystated to the contrary, embodiments “comprising” or “having” an elementor a plurality of elements having a particular property may includeadditional elements not having that property.

Various embodiments provide systems and methods for improving thesensitivity of image acquisition, for example in Nuclear Medicine (NM)imaging applications. Various embodiments provide one or more differentapproaches for improving sensitivity and/or other aspects of detectorperformance. For example, in one approach, an array of pinhole openingsare used in a collimator for a detector system. As another example,additionally or alternatively, in a second approach, all events areidentified as being absorbed at a location and/or within a rangecorresponding to an absorption coefficient of the detector (e.g., oneover the absorption coefficient of the detector). As one more example,in a third approach, all events are identified as being absorbed at alocation and/or within a range that ensures that the energy of theevents is measured within the energy window used for imaging. It may benoted that each of the three approaches discussed above in thisparagraph may be employed with the use of virtual sub-pixels (or virtualdivision of the detector) along X and Y directions (or along the widthand length of the detector).

In various embodiments, a pinhole collimator includes an array ofpinholes that defines multiples cells. Each cell includes or correspondsto only a single physical pixel of the detector and only a singlepinhole of the array. In various embodiments, in contrast to certainconventional approaches, radiation from a given pinhole (also referredto herein as a pinhole opening) only arrives at one particular physicalpixel corresponding to the given pinhole. It may be noted that thephysical pixel may be viewed as including a number of virtual sub-pixelsby a processing unit. Each event detected in the physical pixel iscounted as related to one of the virtual sub-pixels into which thephysical pixel is divided to. The virtual sub-pixel to which the eventbelongs to is determined by the location of the event within thephysical pixel that includes the virtual sub-pixels. There are knownmethods to derive the location of the event within the physical pixel,such as the method described in U.S. patent application Ser. No.15/280,640 entitled “SYSTEMS AND METHODS FOR SUB-PIXEL LOCATIONDETERMINATION” filed Sep. 29, 2016. virtual sub-pixels

Further, use of a pinhole array instead of a parallel hole array invarious embodiments provides for a smaller area of opening that collectsradiation. For example, the size of a pinhole opening may be ⅓ of thewidth (or 1/9 of the area) of a pixel, whereas an opening of a parallelhole array may be the pixel size less the septa thickness. Accordingly,the opening size in a parallel hole array may be dictated by the pixelsize and wall (or septa) thickness; however, in various embodimentsemploying a pinhole array, the opening size may be selected as desired(e.g., to provide a desired sensitivity and/or collimator height). Insome embodiments, physical pixels may be divided in multiple virtualsub-pixels (e.g., an associated processing unit may assign virtualsub-pixels to each physical pixel) along X and Y directions (or lengthand width of a detector), while having a single layer in the Z direction(or thickness). Alternatively, in other embodiments, multiple virtualsub-pixels may be employed along the Z direction (or thickness).

Accordingly, various embodiments provide flexibility to selectedcollimator height. Such flexibility is especially advantageous whenusing a collimator within a swiveling detector head. For example, thecollimator height may dictate or influence the radius reserved for eachhead for swiveling motion. The shorter the collimator is, the smaller isthe required radius. Accordingly, for a shorter collimator provided by apinhole array, more heads may be placed around the object being imagedproviding improved sensitivity and image quality in comparison with ataller parallel hole collimator. Additionally, the collimator openingsproduce solid angles through which the virtual pixels observe the objectbeing imaged, with the smaller size of the pinhole openings providinglarger and more separated solid angles for the virtual sub-pixels, withless overlap between the solid angles for the virtual sub-pixels of aphysical pixel. Accordingly, the use of such pinhole openings improvesspatial resolution, or may be used to maintain a desired resolution witha shorter collimator to increase sensitivity and improve image quality.Further, still, the use of thicker septa or walls helps preventradiation penetration from a given collimator opening to non-associatedpixels (or pixels other than a pixel immediately below the opening),thereby improving image quality. Further still, it may be noted that useof many sub-voxels over the thickness of a detector may reduce thenumber of events for each volume of interest, thereby increasingstatistical noise and degrading image quality. In various embodiments,using only one location (or range) or layer along the Z direction (orthickness) reduces statistical noise and improves image quality.

It may be noted that in some embodiments, in connection withsub-pixelization along the X and Y directions, a single Z layer,location, or range may be used to identify events along a thickness of adetector. For example, a Z position-range for all events may be definedat or around an average absorption depth of 1/μ, where μ is theabsorption coefficient for a specific photon energy for a particulardetector material. For example, events may be distributed linearly, asone example, or exponentially, as another example, within a rangecentered about or otherwise corresponding to distance of 1/μ from thecathode. As another example, a Z position-range for all events may bedefined within a range corresponding to energies of the energy windowused for imaging. For example, in some embodiments, an absorptionlocation for each absorbed photon within the thickness of thesemiconductor detector is defined within a range such that ΔL/D=ΔE/E,where ΔL is a distance from the cathode, D is the detector thickness, ΔEis an energy window width, and E is a photopeak energy of an absorbedphoton. Again, the events may be distributed linearly, as one example,or exponentially, as another example. In various embodiments, use ofsuch Z position-ranges (in contrast, for example, to multiple virtualsub-pixels along a detector thickness) helps to reduce statistical noiseand to improve image quality. Also, it may be noted that use of such Zposition-ranges may be accomplished with simpler hardware or software(in comparison to, for example, use of multiple virtual sub-pixels alonga detector thickness), providing for easier implementation and/or lowercost.

A technical effect provided by various embodiments includes increasedsensitivity of a detector system, such as a NM imaging detector system.The detector system may be provided in a rotating head detector modulethat may be used as part of a group of similar rotating head detectormodules in an imaging system. A technical effect of various embodimentsincludes improved image quality and spatial resolution. A technicaleffect of various embodiments includes reduced collimator heightallowing for less room needed to allow a detector head to pivot,allowing more detector heads to be placed closely to an object beingimaged. A technical effect of various embodiments includes reducedpenetration by radiation to pixels other than a pixel associated with(e.g., located directly below) a collimator opening. A technical effectof various embodiments included reduced statistical noise.

FIG. 1 provides a schematic block view of a detector assembly 100 inaccordance with various embodiments, FIG. 2 provides an exploded view ofaspects of the detector assembly 100, and FIG. 3 provides a sectionalview taken along line 3-3 of FIG. 1. As seen in FIGS. 1-3, the detectorassembly 100 includes a semiconductor detector 110, a pinhole collimator130, and a processing unit 150, which for the clarity of the drawings isshown only in FIG. 1. Generally, the semiconductor detector 110 producessignals in response to absorption events (e.g., photons produced inresponse to a radiopharmaceutical that has been administered to anobject being imaged that impact the semiconductor detector 110). Thesignals are provided to the processing unit 150, which uses identifiedevents to reconstruct an image of the object being imaged and to derivethe location of the event inside the physical pixel as described, forexample in U.S. patent application Ser. No. 15/280,640. The pinholecollimator 130 guides photons to the semiconductor detector 110, andlimits the angular range of approach of photons to a given pixel orportion of the semiconductor detector 110, helping to allow for accuratedetermination of the portion of the object being imaged from which agiven detected event originated.

As best seen in FIG. 3, the semiconductor detector 110 has a firstsurface 112 and a second surface 114. The second surface 114 is opposedto the first surface 112 (and, likewise, the first surface 112 isopposed to the second surface 114). The semiconductor detector 110 isconfigured to generate electrical signal in response to photon impacts,and may be made of, for example, Cadmium Zinc Telluride (CZT). Thesecond surface 114 includes pixelated anodes 116 disposed thereon, andthe first surface 112 includes a cathode electrode 118 disposed thereon.In some embodiments, the cathode electrode 118 may be a monolithic, orsingle, cathode. The cathode electrode 118 collects an oppositeelectrical charge of the pixelated anodes 116, and the pixelated anodes116 are used to generate signals in response to charges generated by thesemiconductor detector 110 responsive to photon impacts. The pixelatedanodes 116 may be arranged in a grid, with the location of one or morepixelated anodes 116 at which a signal is generated responsive to aphoton impact used to determine a corresponding location in the objectcorresponding to the photon impact.

As seen in FIGS. 1-3, the pinhole collimator 130 is interposed betweenthe semiconductor detector 110 and an object being imaged (not shown),and is used to control passage of radiation from the object being imagedto the semiconductor detector 110 via the pinhole collimator 130. Forexample, the pinhole collimator 130 guides photons to the semiconductordetector 110, limiting an angular range of approach for photons thatimpact the semiconductor detector 110. The pinhole collimator 130includes an array 132 of pinhole openings 134 corresponding to thepixelated anodes 116 on the second surface 114. In the illustratedembodiment, the array 132 of pinhole openings 134 has a 1:1correspondence with an array or grid of pixelated anodes 116, with boththe pixelated anodes 116 and array 132 of pinhole openings 134 arrangedin an 8×8 layout when the projections of openings 132 on the secondsurface 114 of the detector 110 are centered in the pixelated anodes116. Accordingly, in the illustrated embodiment, each pinhole opening134 is associated with a single pixelated anode 116 of the semiconductordetector 110. Accordingly, radiation that passes through a given pinholeopening 134 is confined within a single cell of collimator 130 and isabsorbed at a location corresponding to one and only one pixelated anode116 that is associated with the given pinhole opening 134 (e.g., locateddirectly beneath the pinhole opening 134). Further, each pinhole opening134 defines an area, with the area of each pinhole opening 134 smallerthan a corresponding area of the corresponding pixelated anode 116. Forexample, as seen in FIG. 3, the width of each pixelated anode 116 isgreater than the width of a corresponding pinhole opening 134.Accordingly, if the pixelated anode 116 and pinhole opening 134 aregenerally square-shaped, the area of the pixelated anode 116 is greaterthan the area of the pinhole opening 134. It may be noted that thedepicted examples have generally square-shaped cross-sections. Othershapes of opening (e.g., circular, rectangular, or triangular, amongothers), may be utilized in alternate embodiments.

As best seen in FIGS. 1 and 3, in various embodiments the pinholecollimator 130 includes a top plate 140 through which the pinholeopenings 134 pass. The top plate 140 is mounted to a collimator base131. The pinhole collimator 130 also includes plural septa 142 (orwalls) extending along a height of the collimator base 131 that definecollimator cells 144 corresponding to the pinhole openings 134. In theillustrated embodiment, each pinhole opening 134 is associated with aparticular collimator cell 144 and a particular pixelated anode 116,with photons that pass through the pinhole opening 134 passing throughthe corresponding collimator cell 144 toward the corresponding pixelatedanode 116. Each collimator cell 144 defines a cavity between thecorresponding pinhole opening 134 and the corresponding pixelated anode116. The septa 142 act to reduce or eliminate passage of a photonthrough a pinhole opening 134 to non-corresponding pixelated anodes(e.g., pixelated anodes adjacent to the particular pixelated anode thatcorresponds to the particular pinhole opening). A cell width 148 definedby the septa 142 (e.g., a width between neighboring septa 142) isgreater than an opening width 146 defined by the pinhole openings 134.For example, in some embodiments, the cell width 148 is 3 times or moregreater than the opening width 146. In the example illustrated in FIGS.1 to 3, the cell width 148 is d, and the opening width 146 is d/3, orthe cell width 148 is 3 times greater than the opening width 146. It maybe noted that, in contrast, an opening width and width betweenneighboring walls may be identical for a parallel-hole collimator.

In the example illustrated in FIGS. 1 to 3, the septa 142 are parallelto each other and define square-shaped cross sections for eachcollimator cell 144; however, it may be noted that differentconfigurations may be employed in alternate embodiments. As best seen inFIGS. 2 and 3, the top plate 140 has a thickness 141, and the septa 142have a thickness 143. In the depicted example, the thickness 141 of thetop plate 140 is greater than the thickness 143 of the septa 142. In theexample illustrated in FIGS. 1 to 3, the septa 142 are parallel to eachother and define square-shaped cross sections for each collimator cell144; however, it may be noted that different configurations may beemployed in alternate embodiments.

FIG. 4 illustrates a cross-section of an example embodiment of a topplate 400 (e.g., which may be used as top plate 140) that may be usedwith pinhole collimator 130 in various embodiments. The top plate 400includes a first surface 402 configured to be positioned proximate asemiconductor detector (e.g., semiconductor detector 110), or orientedtoward an interior 410 of a collimator (e.g., pinhole collimator 130)including the top plate 400. The top plate 400 also includes a secondsurface 404 that is opposed to the first surface 402. The second surface404 is farther away from the semiconductor detector than the firstsurface 402 is, or the second surface 404 is oriented toward an object420 being imaged from which photons 422 are emitted. The top plate 400includes pinhole openings 430 through which photons 422 pass toward thesemiconductor detector. The depicted pinhole openings 430 each have afirst width 432 at the first surface 402, and a second width 434 at thesecond surface 404. The first width 432 is greater than the second width434. Accordingly, the pinhole openings 430 are tapered, and are largerat the first surface 402 than at the second surface 404. It may be notedthat the taper orientation of the openings 430 in the plate 400 of FIG.4 is opposite to the taper orientation of the openings 134 in the plate140 of FIGS. 1-3. The tapered shape in various embodiments is configuredto facilitate passage of photons over a preferred or desired angularrange.

Alternatively or additionally, it may be noted that pinhole collimatorsin various embodiments may include tapered walls. FIG. 5 illustrates across-section of an example embodiment of pinhole collimator septa 500that may be used with pinhole collimator 130 in various embodiments.Septa 500 include a first surface 502 proximate to a top plate (e.g.,top plate 141, top plate 400; top plate not shown in FIG. 5), and asecond surface 504 proximate to a semiconductor detector (e.g.,semiconductor detector 110; semiconductor detector not shown in FIG. 5).Cells 506, through which photons pass, are defined between neighboringsepta 500. The cells 506 are a first width 512 at the first surface 502,and a second width 514 at the second surface 504, with the second width514 greater than the first width 512. Accordingly, a first width 503 ofthe septa 500 at the first surface 502 is greater than a second width505 of the septa 500 at the second surface 504. In the illustratedembodiment, a pitch 530 is defined by the septa 500, with the firstwidth 512 less than the pitch 530. The tapered septa 500 in variousembodiments may cooperate or be complementary with tapered openings(e.g., openings 434). In various embodiments, the tapered septa 500 maybe formed by 3D printing a collimator block. The tapered septa 500provide additional thickness (e.g., relative to septa thickness of aparallel hole collimator) for improved reduction of penetration byphotons into adjacent collimator cells. The tapered shape in variousembodiments is configured to facilitate passage of photons over apreferred or desired angular range. It may be noted that while thedescription above includes top plate 141 or 400, the pinholes-arraycollimator of FIG. 5 may not include a top plate at all when opening 512is in the desired size of the pinholes openings such as the size ofopenings 134 and 434 of FIGS. 4 and 1-3, respectively.

Examples of solid angles corresponding to the pinhole collimator 130 maybe seen in FIG. 3. As seen in FIG. 3, the solid angles defined byvirtual sub-pixels may vary based on a depth (or depths) within thesemiconductor detector 110 assigned to events. For example, solid angles320 a, 320 b, and 320 c result from using a common absorption depth(e.g., 1/μ, where μ is an absorption coefficient) for events from threeadjacent virtual sub-pixels 117. As another example, solid angles 330 a,330 b, and 330 c result from using varying absorption depths for eventsfrom three adjacent virtual sub-pixels 117. In FIG. 3, the cell width148 between septa 142 of the pinhole collimator equals d, a height 149of the pinhole collimator 130 is h/3, and the opening width 146 definedby the pinhole openings 134 is d/3.

By way of comparison, examples of solid angles corresponding to use of aparallel hole collimator may be seen in FIG. 6. In FIG. 6, a detectorsystem 600 includes a parallel hole collimator 602 that includes walls610 having a common opening width 612 therebetween. The opening width612 defines the width of openings 620. The detector system 600 alsoincludes a detector unit 621 that includes pixelated anodes 616 havingvirtual sub-pixels 617 associated therewith (e.g., by a processingunit). As seen in FIG. 6, solid angles 660 a, 660 b, and 660 c resultfrom using a common absorption depth (e.g., at the surface of thedetector unit 621) for events from three adjacent virtual sub-pixels617. As seen in FIGS. 3 and 6, the solid angles (solid angles 320 a, 320b, and 320 c and/or solid angles 330 a, 330 b, and 330 c) for thepinhole collimator 130 have noticeably less overlap than the solidangles for parallel hole collimator 602 (solid angles 660 a, 660 b, and660 c). In FIG. 6, the width 612 between walls (as well as width ofopenings 620) is d, and the height 652 of the parallel hole collimator602 is h.

Returning to FIGS. 1-3, the processing unit 150 is operably coupled tothe semiconductor detector 110, and is configured to identify detectedevents, deriving the location of events within physical pixels 119 andbased on their location, assigning them to virtual sub-pixels 117distributed along a length 190 and width 191 of the semiconductordetector 110 to be counted there. In FIG. 3, the virtual sub-pixels 117are represented by dashed lines passing through the semiconductordetector corresponding to the location of the virtual sub-pixels 117. Itmay be noted that in the illustrated embodiment, the semiconductordetector 110 includes pixels 119. In FIG. 3, there are 3 virtualsub-pixels across a width of each pixel 119. Each pixel 119 may beunderstood as including a pixelated anode 116, with each pixelated anode116 smaller (having a smaller area) than the corresponding pixel 119. Inthe illustrated embodiment, there are 9 virtual sub-pixels 117 perpixelated anode 116 or pixel 119 (e.g., a grid of 3×3 virtual sub-pixels117 per pixelated anode 116 or pixel 119). Each pixel 119 includes aplurality of corresponding virtual sub-pixels, with absorbed photons inthe semiconductor detector 110 counted as events in a correspondingvirtual sub-pixel. Additional discussion regarding virtual sub-pixelsand the use of virtual sub-pixels, and the use of collected andnon-collected charge signals may be found in U.S. patent applicationSer. No. 14/724,022, entitled “Systems and Method for Charge-SharingIdentifcation and Correction Using a Single Pixel,” filed 28 May 2015(“the 022 Application); U.S. patent application Ser. No. 15/280,640,entitled “Systems and Methods for Sub-Pixel Location Determination,”filed 29 Sep. 2016 (“the 640 Application”); and U.S. patent applicationSer. No. 14/627,436, entitled “Systems and Methods for Improving EnergyResolution by Sub-Pixel Energy Calibration,” filed 20 Feb. 2015 (“the436 Application). The subject matter of each of the 022 Application, the640 Application, and the 436 Application are incorporated by referencein its entirety.

In various embodiments the processing unit 150 includes processingcircuitry configured to perform one or more tasks, functions, or stepsdiscussed herein. It may be noted that “processing unit” as used hereinis not intended to necessarily be limited to a single processor orcomputer. For example, the processing unit 150 may include multipleprocessors, ASIC's, FPGA's, and/or computers, which may be integrated ina common housing or unit, or which may distributed among various unitsor housings. It may be noted that operations performed by the processingunit 150 (e.g., operations corresponding to process flows or methodsdiscussed herein, or aspects thereof) may be sufficiently complex thatthe operations may not be performed by a human being within a reasonabletime period. For example, the determination of values of collected,non-collected, and/or combined charge signals within the timeconstraints associated with such signals may rely on or utilizecomputations that may not be completed by a person within a reasonabletime period.

As discussed, herein, signals are generated by one or more pixelatedanodes 116 in response to a photon impact, with the location of thepixelated anode(s) 116 generating a signal used to determine acorresponding location in the object for which an event is counted. Invarious embodiments, as also discussed in the 022 Application, the 640Application, and the 436 Application, signals from adjacent pixels maybe used to assign a virtual sub-pixel location within a given pixelatedanode 116. In some embodiments, the processing unit 150 is configured todetermine an absorption location for a given absorbed phon based onnon-collected signals received from pixelated anodes adjacent to apixelated anode absorbing the given absorbed photon.

Additionally or alternatively to the use of virtual pixels along alength and/or width of the semiconductor detector 110, in variousembodiments virtual pixels may be employed along a thickness of thesemiconductor detector 110. Virtual pixels employed along a thickness ofthe semiconductor detector 110 may be used to represent different depthsof absorption of photons. For example, in various embodiments, as bestseen in FIG. 3, the semiconductor detector 110 has a thickness 396.Three rows of virtual pixels are distributed along the thickness 396—afirst row 390, a second row 392, and a third row 394. The processingunit 150 in various embodiments is configured to identify detectorevents with the virtual sub-pixels in the first row 390, second row 392,and third row 394 distributed along the thickness 396. Accordingly, invarious embodiments, different virtual sub-pixels along a thickness maybe used to provide different absorption depths for identifying eventlocations. For example, as seen in FIG. 3, event 331 a is shown at adepth corresponding to the first row 390, event 331 b is shown at adepth corresponding to the third row 394, and event 331 c is shown at adepth corresponding to the second row 392.

However, it may be noted that, in other embodiments that may or may notinclude a pinhole-array collimator, a single absorption depth may beemployed. For example, in some embodiments, the processing unit 150 isconfigured to count absorbed photons as events within the thickness 396of the semiconductor detector 110 at a location (e.g., a distance fromthe cathode 118) corresponding to one over an absorption coefficient ofthe semiconductor detector 110. For example, with μ as the absorptioncoefficient, photons (e.g., photons at a given energy corresponding withthe absorption coefficient) may be counted as events at a location inthe semiconductor detector a distance 395 from the second surface 112(and/or cathode 118) along the thickness 396, as shown for eventlocations 321 a, 321 b, and 321 c of FIG. 3. The distance 395 in variousembodiments is 1/μ. It may be noted that μ may vary based on photonenergy. It may further be noted that use of a single absorption depth asdiscussed herein and in the next paragraph may be used in connectionwith a pinhole collimator (e.g., pinhole collimator 130) in variousembodiments, or may be used in connection with a parallel holecollimator (e.g., parallel hole collimator 602) in other embodiments. Insome embodiments, the absorption location for each photon is definedwithin a range of 1/μ±1 millimeter.

As another example of use of a single absorption depth, in someembodiments, the processing unit 150 is configured to count absorbedphotons as events within the thickness 396 of the semiconductor detector110 at a distance corresponding to an energy window width used toidentify the events as photon impacts. For example, in some embodiments,an absorption location for each absorbed photon within the thickness 396of the semiconductor detector 110 is defined within a range such thatΔL/D=ΔE/E, where ΔL is the distance 395 from the first surface 112(and/or the cathode 118), D is the detector thickness (e.g., thickness396), ΔE is an energy window width, and E is a photopeak energy of anabsorbed photon. The energy window width in various embodiments is arange of energies around the photopeak energy which are considered astrue events.

FIG. 7 is a schematic illustration of a NM imaging system 1000 having aplurality of imaging detector head assemblies mounted on a gantry (whichmay be mounted, for example, in rows, in an iris shape, or otherconfigurations, such as a configuration in which the movable detectorcarriers 1016 are aligned radially toward the patient-body 1010). Inparticular, a plurality of imaging detectors 1002 are mounted to agantry 1004. Each detector 1002 may include, for example, collimatorsand detectors arranged generally similarly to the arrangements discussedin connection with FIGS. 1-6. In the illustrated embodiment, the imagingdetectors 1002 are configured as two separate detector arrays 1006 and1008 coupled to the gantry 1004 above and below a subject 1010 (e.g., apatient), as viewed in FIG. 7. The detector arrays 1006 and 1008 may becoupled directly to the gantry 1004, or may be coupled via supportmembers 1012 to the gantry 1004 to allow movement of the entire arrays1006 and/or 1008 relative to the gantry 1004 (e.g., transversetranslating movement in the left or right direction as viewed by arrow Tin FIG. 7). Additionally, each of the imaging detectors 1002 includes adetector unit 1014 (which may include collimator and/or detectorassemblies as discussed herein in connection with FIGS. 1-6), at leastsome of which are mounted to a movable detector carrier 1016 (e.g., asupport arm or actuator that may be driven by a motor to cause movementthereof) that extends from the gantry 1004. In some embodiments, thedetector carriers 1016 allow movement of the detector units 1014 towardsand away from the subject 1010, such as linearly. Thus, in theillustrated embodiment the detector arrays 1006 and 1008 are mounted inparallel above and below the subject 1010 and allow linear movement ofthe detector units 1014 in one direction (indicated by the arrow L),illustrated as perpendicular to the support member 1012 (that arecoupled generally horizontally on the gantry 1004). However, otherconfigurations and orientations are possible as described herein. Itshould be noted that the movable detector carrier 1016 may be any typeof support that allows movement of the detector units 1014 relative tothe support member 1012 and/or gantry 1004, which in various embodimentsallows the detector units 1014 to move linearly towards and away fromthe support member 1012.

Each of the imaging detectors 1002 in various embodiments is smallerthan a conventional whole body or general purpose imaging detector. Aconventional imaging detector may be large enough to image most or allof a width of a patient's body at one time and may have a diameter or alarger dimension of approximately 50 cm or more. In contrast, each ofthe imaging detectors 1002 may include one or more detector units 1014coupled to a respective detector carrier 1016 and having dimensions of,for example, 4 cm to 20 cm and may be formed of Cadmium Zinc Telluride(CZT) tiles or modules. For example, each of the detector units 1014 maybe 8×8 cm in size and be composed of a plurality of CZT pixelatedmodules (not shown). For example, each module may be 4×4 cm in size andhave 16×16=256 pixels. In some embodiments, each detector unit 1014includes a plurality of modules, such as an array of 1×7 modules.However, different configurations and array sizes are contemplatedincluding, for example, detector units 1014 having multiple rows ofmodules.

It should be understood that the imaging detectors 1002 may be differentsizes and/or shapes with respect to each other, such as square,rectangular, circular or other shape. An actual field of view (FOV) ofeach of the imaging detectors 1002 may be directly proportional to thesize and shape of the respective imaging detector.

The gantry 1004 may be formed with an aperture 1018 (e.g., opening orbore) therethrough as illustrated. A patient table 1020, such as apatient bed, is configured with a support mechanism (not shown) tosupport and carry the subject 1010 in one or more of a plurality ofviewing positions within the aperture 1018 and relative to the imagingdetectors 1002. Alternatively, the gantry 1004 may comprise a pluralityof gantry segments (not shown), each of which may independently move asupport member 1012 or one or more of the imaging detectors 1002.

The gantry 1004 may also be configured in other shapes, such as a “C”,“H” and “L”, for example, and may be rotatable about the subject 1010.For example, the gantry 1004 may be formed as a closed ring or circle,or as an open arc or arch which allows the subject 1010 to be easilyaccessed while imaging and facilitates loading and unloading of thesubject 1010, as well as reducing claustrophobia in some subjects 1010.

Additional imaging detectors (not shown) may be positioned to form rowsof detector arrays or an arc or ring around the subject 1010. Bypositioning multiple imaging detectors 1002 at multiple positions withrespect to the subject 1010, such as along an imaging axis (e.g., headto toe direction of the subject 1010) image data specific for a largerFOV may be acquired more quickly.

Each of the imaging detectors 1002 has a radiation detection face, whichis directed towards the subject 1010 or a region of interest within thesubject.

In various embodiments, multi-bore collimators may be constructed to beregistered with pixels of the detector units 1014, which in oneembodiment are CZT detectors. However, other materials may be used.Registered collimation may improve spatial resolution by forcing photonsgoing through one bore to be collected primarily by one pixel.Additionally, registered collimation may improve sensitivity and energyresponse of pixelated detectors as detector area near the edges of apixel or in-between two adjacent pixels may have reduced sensitivity ordecreased energy resolution or other performance degradation. Havingcollimator septa directly above the edges of pixels reduces the chanceof a photon impinging at these degraded-performance locations, withoutdecreasing the overall probability of a photon passing through thecollimator. As discussed herein, in various embodiments parallel-holeand/or pin-hole collimation may be employed.

A controller unit 1030 may control the movement and positioning of thepatient table 1020, imaging detectors 1002 (which may be configured asone or more arms), gantry 1004 and/or the collimators 1022 (that movewith the imaging detectors 1002 in various embodiments, being coupledthereto). A range of motion before or during an acquisition, or betweendifferent image acquisitions, is set to maintain the actual FOV of eachof the imaging detectors 1002 directed, for example, towards or “aimedat” a particular area or region of the subject 1010 or along the entiresubject 1010. The motion may be a combined or complex motion in multipledirections simultaneously, concurrently, or sequentially as described inmore detail herein.

The controller unit 1030 may have a gantry motor controller 1032, tablecontroller 1034, detector controller 1036, pivot controller 1038, andcollimator controller 1040. The controllers 1030, 1032, 1034, 1036,1038, 1040 may be automatically commanded by a processing unit 1050,manually controlled by an operator, or a combination thereof. The gantrymotor controller 1032 may move the imaging detectors 1002 with respectto the subject 1010, for example, individually, in segments or subsets,or simultaneously in a fixed relationship to one another. For example,in some embodiments, the gantry controller 1032 may cause the imagingdetectors 1002 and/or support members 1012 to move relative to or rotateabout the subject 1010, which may include motion of less than or up to180 degrees (or more).

The table controller 1034 may move the patient table 1020 to positionthe subject 1010 relative to the imaging detectors 1002. The patienttable 1020 may be moved in up-down directions, in-out directions, andright-left directions, for example. The detector controller 1036 maycontrol movement of each of the imaging detectors 1002 to move togetheras a group or individually as described in more detail herein. Thedetector controller 1036 also may control movement of the imagingdetectors 1002 in some embodiments to move closer to and farther from asurface of the subject 1010, such as by controlling translating movementof the detector carriers 1016 linearly towards or away from the subject1010 (e.g., sliding or telescoping movement). Optionally, the detectorcontroller 1036 may control movement of the detector carriers 1016 toallow movement of the detector array 1006 or 1008. For example, thedetector controller 1036 may control lateral movement of the detectorcarriers 1016 illustrated by the T arrow (and shown as left and right asviewed in FIG. 7). In various embodiments, the detector controller 1036may control the detector carriers 1016 or the support members 1012 tomove in different lateral directions. Detector controller 1036 maycontrol the swiveling motion of detectors 1002 together with theircollimators 1022.

The pivot controller 1038 may control pivoting or rotating movement ofthe detector units 1014 at ends of the detector carriers 1016 and/orpivoting or rotating movement of the detector carrier 1016. For example,one or more of the detector units 1014 or detector carriers 1016 may berotated about at least one axis to view the subject 1010 from aplurality of angular orientations to acquire, for example, 3D image datain a 3D SPECT or 3D imaging mode of operation. The collimator controller1040 may adjust a position of an adjustable collimator, such as acollimator with adjustable strips (or vanes) or adjustable pinhole(s).

It should be noted that motion of one or more imaging detectors 1002 maybe in directions other than strictly axially or radially, and motions inseveral motion directions may be used in various embodiment. Therefore,the term “motion controller” may be used to indicate a collective namefor all motion controllers. It should be noted that the variouscontrollers may be combined, for example, the detector controller 1036and pivot controller 1038 may be combined to provide the differentmovements described herein.

Prior to acquiring an image of the subject 1010 or a portion of thesubject 1010, the imaging detectors 1002, gantry 1004, patient table1020 and/or collimators 1022 may be adjusted, such as to first orinitial imaging positions, as well as subsequent imaging positions. Theimaging detectors 1002 may each be positioned to image a portion of thesubject 1010. Alternatively, for example in a case of a small sizesubject 1010, one or more of the imaging detectors 1002 may not be usedto acquire data, such as the imaging detectors 1002 at ends of thedetector arrays 1006 and 1008, which as illustrated in FIG. 7 are in aretracted position away from the subject 1010. Positioning may beaccomplished manually by the operator and/or automatically, which mayinclude using, for example, image information such as other imagesacquired before the current acquisition, such as by another imagingmodality such as X-ray Computed Tomography (CT), MRI, X-Ray, PET orultrasound. In some embodiments, the additional information forpositioning, such as the other images, may be acquired by the samesystem, such as in a hybrid system (e.g., a SPECT/CT system).Additionally, the detector units 1014 may be configured to acquirenon-NM data, such as x-ray CT data. In some embodiments, amulti-modality imaging system may be provided, for example, to allowperforming NM or SPECT imaging, as well as x-ray CT imaging, which mayinclude a dual-modality or gantry design as described in more detailherein.

After the imaging detectors 1002, gantry 1004, patient table 1020,and/or collimators 1022 are positioned, one or more images, such asthree-dimensional (3D) SPECT images are acquired using one or more ofthe imaging detectors 1002, which may include using a combined motionthat reduces or minimizes spacing between detector units 1014. The imagedata acquired by each imaging detector 1002 may be combined andreconstructed into a composite image or 3D images in variousembodiments.

In one embodiment, at least one of detector arrays 1006 and/or 1008,gantry 1004, patient table 1020, and/or collimators 1022 are moved afterbeing initially positioned, which includes individual movement of one ormore of the detector units 1014 (e.g., combined lateral and pivotingmovement) together with the swiveling motion of detectors 1002. Forexample, at least one of detector arrays 1006 and/or 1008 may be movedlaterally while pivoted. Thus, in various embodiments, a plurality ofsmall sized detectors, such as the detector units 1014 may be used for3D imaging, such as when moving or sweeping the detector units 1014 incombination with other movements.

In various embodiments, a data acquisition system (DAS) 1060 receiveselectrical signal data produced by the imaging detectors 1002 andconverts this data into digital signals for subsequent processing.However, in various embodiments, digital signals are generated by theimaging detectors 1002. An image reconstruction device 1062 (which maybe a processing device or computer) and a data storage device 1064 maybe provided in addition to the processing unit 1050. It should be notedthat one or more functions related to one or more of data acquisition,motion control, data processing and image reconstruction may beaccomplished through hardware, software and/or by shared processingresources, which may be located within or near the imaging system 1000,or may be located remotely. Additionally, a user input device 1066 maybe provided to receive user inputs (e.g., control commands), as well asa display 1068 for displaying images. DAS 1060 receives the acquiredimages from detectors 1002 together with the corresponding lateral,vertical, rotational and swiveling coordinates of gantry 1004, supportmembers 1012, detector units 1014, detector carriers 1016, and detectors1002 for accurate reconstruction of an image including 3D images andtheir slices.

It should be noted that the various embodiments may be implemented inhardware, software or a combination thereof. The various embodimentsand/or components, for example, the modules, or components andcontrollers therein, also may be implemented as part of one or morecomputers or processors. The computer or processor may include acomputing device, an input device, a display unit and an interface, forexample, for accessing the Internet. The computer or processor mayinclude a microprocessor. The microprocessor may be connected to acommunication bus. The computer or processor may also include a memory.The memory may include Random Access Memory (RAM) and Read Only Memory(ROM). The computer or processor further may include a storage device,which may be a hard disk drive or a removable storage drive such as asolid-state drive, optical disk drive, and the like. The storage devicemay also be other similar means for loading computer programs or otherinstructions into the computer or processor.

As used herein, the term “computer” or “module” may include anyprocessor-based or microprocessor-based system including systems usingmicrocontrollers, reduced instruction set computers (RISC), ASICs, logiccircuits, and any other circuit or processor capable of executing thefunctions described herein. The above examples are exemplary only, andare thus not intended to limit in any way the definition and/or meaningof the term “computer”.

The computer or processor executes a set of instructions that are storedin one or more storage elements, in order to process input data. Thestorage elements may also store data or other information as desired orneeded. The storage element may be in the form of an information sourceor a physical memory element within a processing machine.

The set of instructions may include various commands that instruct thecomputer or processor as a processing machine to perform specificoperations such as the methods and processes of the various embodiments.The set of instructions may be in the form of a software program. Thesoftware may be in various forms such as system software or applicationsoftware and which may be embodied as a tangible and non-transitorycomputer readable medium. Further, the software may be in the form of acollection of separate programs or modules, a program module within alarger program or a portion of a program module. The software also mayinclude modular programming in the form of object-oriented programming.The processing of input data by the processing machine may be inresponse to operator commands, or in response to results of previousprocessing, or in response to a request made by another processingmachine.

As used herein, a structure, limitation, or element that is “configuredto” perform a task or operation is particularly structurally formed,constructed, or adapted in a manner corresponding to the task oroperation. For purposes of clarity and the avoidance of doubt, an objectthat is merely capable of being modified to perform the task oroperation is not “configured to” perform the task or operation as usedherein. Instead, the use of “configured to” as used herein denotesstructural adaptations or characteristics, and denotes structuralrequirements of any structure, limitation, or element that is describedas being “configured to” perform the task or operation. For example, aprocessing unit, processor, or computer that is “configured to” performa task or operation may be understood as being particularly structuredto perform the task or operation (e.g., having one or more programs orinstructions stored thereon or used in conjunction therewith tailored orintended to perform the task or operation, and/or having an arrangementof processing circuitry tailored or intended to perform the task oroperation). For the purposes of clarity and the avoidance of doubt, ageneral purpose computer (which may become “configured to” perform thetask or operation if appropriately programmed) is not “configured to”perform a task or operation unless or until specifically programmed orstructurally modified to perform the task or operation.

As used herein, the terms “software” and “firmware” are interchangeable,and include any computer program stored in memory for execution by acomputer, including RAM memory, ROM memory, EPROM memory, EEPROM memory,and non-volatile RAM (NVRAM) memory. The above memory types areexemplary only, and are thus not limiting as to the types of memoryusable for storage of a computer program.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, the above-describedembodiments (and/or aspects thereof) may be used in combination witheach other. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the variousembodiments without departing from their scope. While the dimensions andtypes of materials described herein are intended to define theparameters of the various embodiments, they are by no means limiting andare merely exemplary. Many other embodiments will be apparent to thoseof skill in the art upon reviewing the above description. The scope ofthe various embodiments should, therefore, be determined with referenceto the appended claims, along with the full scope of equivalents towhich such claims are entitled. In the appended claims, the terms“including” and “in which” are used as the plain-English equivalents ofthe respective terms “comprising” and “wherein.” Moreover, in thefollowing claims, the terms “first,” “second,” and “third,” etc. areused merely as labels, and are not intended to impose numericalrequirements on their objects. Further, the limitations of the followingclaims are not written in means-plus-function format and are notintended to be interpreted based on 35 U.S.C. § 112(f) unless and untilsuch claim limitations expressly use the phrase “means for” followed bya statement of function void of further structure.

This written description uses examples to disclose the variousembodiments, including the best mode, and also to enable any personskilled in the art to practice the various embodiments, including makingand using any devices or systems and performing any incorporatedmethods. The patentable scope of the various embodiments is defined bythe claims, and may include other examples that occur to those skilledin the art. Such other examples are intended to be within the scope ofthe claims if the examples have structural elements that do not differfrom the literal language of the claims, or the examples includeequivalent structural elements with insubstantial differences from theliteral languages of the claims.

What is claimed is:
 1. A detector assembly including: a semiconductordetector having a first surface and a second surface opposed to eachother, the first surface including pixels comprising pixelated anodes,and the second surface comprising a cathode electrode; a collimatorincluding openings, each opening associated with a single correspondingpixel of the semiconductor detector; and a processing unit configured toidentify detected events within virtual sub-pixels distributed along alength and width of the semiconductor detector, wherein each pixelcomprises a plurality of corresponding virtual sub-pixels, whereinabsorbed photons are counted as events in a corresponding virtualsub-pixel, wherein absorbed photons are counted as events within athickness of the semiconductor detector at a distance corresponding toan energy window width used to identify the events as photon impacts. 2.The detector assembly of claim 1, wherein an absorption location foreach absorbed photon within the thickness of the semiconductor detectoris defined within a range such that ΔL/D=ΔE/E, where ΔL is a distancefrom the cathode, D is the detector thickness, ΔE is an energy windowwidth, and E is a photopeak energy of an absorbed photon.
 3. Thedetector assembly of claim 1, wherein the processing unit is configuredto determine an absorption location for a given absorbed photon based onnon-collected signals received from pixelated anodes adjacent to apixelated anode absorbing the given absorbed photon.
 4. The detectorassembly of claim 1, wherein the collimator is a pinhole collimator. 5.The detector assembly of claim 1, wherein the collimator is aparallel-hole collimator.
 6. The detector assembly of claim 1, whereinthe events for each pixel are counted at a single shared location. 7.The detector assembly of claim 1, wherein the events for each pixel arecounted within a single shared range.
 8. The detector assembly of claim1, wherein each pixel comprises virtual sub-pixels along the length andwidth, but not along the thickness of the semiconductor detector.